The present invention relates to a videotex system provided with means for protecting it against transmission errors. It can be used in the transmission and display of data on television receivers for random entertainment, information or instructional purposes.
The invention is more particularly applicable to the ANTIOPE system (digital acquisition and television transmission of pictures organised in written page form) which is a transmitted videotex system (i.e. unidirectional) making it possible to insert alphanumeric data organised into pages and magazines into television channels. In such a system, the transmission of data takes place by means and a procedure called DIDON (digital data transmission). This procedure is compatible with the transmission of the television signal.
Numerous articles or patent applications already describe this system. A detailed description is provided more particularly in the following documents:
The article by Y. GUINET entitled "Comparative study of teletext systems in radio transmission. Some advantages of the transmission of data in packets applied to the teletext" published in the Journal U.E.R. cahier Technique, No. 165, October 1977, pp. 242 to 253.
the article by B. MARTI and M. MAUDUIT entitled "ANTIOPE, teletext service" published in the Journal "Radiodiffusion Television", Vol. 9, No. 40, November/December 1975, 5/5, pp. 18-23.
the specification of the ANTIOPE teletext system published by the Centre Commun d/Etudes de Television et Telecommunications (CCETT).
French Patent Application No. 75 18319 filed on June 6th 1975 and entitled "Data transmission system".
French Patent Application No. 76 292, filed on Sept. 6th 1976 and entitled "Digital transmission system and the display of the text on a television screen".
French Patent Application No. 76 29034 filed on Sept. 22nd 1976 and entitled "Improvements to systems for the digital transmission and display of texts on a television screen".
Application for French Certificate of Addition No. 77 17625, filed on June 3rd 1977 and entitled "data transmission system".
The ANTIOPE system will not be described in detail here because it is widely known. However, in order to facilitate the understanding of the invention, its basic principles will be described. For details of its design or construction, reference can be made to the aforementioned documents, which themselves refer to other documents and all these documents are considered as being incorporated into the present description.
FIG. 1 diagrammatically shows the essential elements of an ANTIOPE videotex system. Such a system comprises a transmission station 2 and receiver stations 4, 4', etc. The transmission station comprises:
Means 6 for composing or setting a magazine formed by pages organised into rows of characters. This means is supplied by a data bank which is, for example, a meteorological service, the Stock Exchange, an information agency, etc. PA1 A circuit 8 for the processing and storage of the magazine in the form of digital signals grouped into octets (8 bits) divided up into data octets and control octets. A junction 10 connected to circuit 8. PA1 A multiplexer 12 for the formation of groups of octets and the insertion of data into the lines of a television signal, said means realising the DIDON procedure referred to hereinbefore. PA1 A modulation member 14. PA1 Finally, an antenna 16. PA1 Each receiver station or terminal comprises: PA1 1--page loss PA1 2--page interference PA1 3--incomplete page PA1 4--row interference PA1 5--row loose framing PA1 6--incorrect visual display attribute (colour, size, alphabet, etc) PA1 7--error symbol display PA1 8--erroneous symbol display. PA1 (a) for recognising in these said sequences a particular code despite any minor errors in said sequence and to restore a corrected control code corresponding to the conventional code which is then taken into account by the receiver, PA1 (b) and to recognise and indicate that certain control code sequences are seriously incorrect and cannot be taken into account, PA1 to correct control sequences of two erroneous codes comprising an octet and a single invalid octet in the sense of the parity (octet X), PA1 to correct the control sequences of three erroneous codes comprising up to two octets X without an octet Z, Z being a valid octet in the sense of the parity, but not belonging to the functioning sequence, PA1 to indicate erroneous sequences comprising a number of octets X and octets Z which exceeds a given value, dependent on the sequence length or which comprises one octet X and one octet Z. PA1 to recognise sequences such as: EQU (SOH; RS), (X; RS), (SOH; X) PA1 and to restore in the three cases the conventional page slag code RS, PA1 to recognise that there is an error in sequences such as: EQU (X; X), (Z; RS), (SOH; Z). PA1 to recognise sequences such as: EQU (ETX; EOT), (ETX; X), (X; EOT) PA1 and to restore in the three cases the conventional end of page code ETZ, PA1 to recognise that there is an error in sequences such as: EQU (X; XO, (Z; EOT), ETZ; Z). PA1 to recognise sequences PA1 and to restore in the ten cases the conventional start of row code, PA1 to recognise whether there is an error in sequences incorporating (X; X; X) or "X and Z". PA1 (A) in the transmitter, the sequence formation circuit is able to constitute, for certain visual display attributes, a sequence of four codes: code ESC followed by a code notated Y, all being duplicated, i.e. (ESC; Y; ESC; Y) PA1 recognise the sequences (ESC; Y; ESC; Y), (ESC; X; ESC; Y), (ESC; Y; X; Y) and (X; Y; ESC; Y) and to restore in the four cases the conventional code (ESC; Y), PA1 to recognise that there is an error in sequences such as: EQU (X; X; ESC; Y)
A circuit 20 for the reception and demodulation of a television signal.
A digital signal processing channel 22.
A picture video signal processing channel 28.
A visual display member 30.
In the ANTIOPE system, the octets carrying the data are conventionally notated in the following manner: ##STR1##
Bit b.sub.8 is an oddness element, or in other words an element such that the total number of "1" appearing in the octet is odd. According to this notation, the octet (j/k)=(4, 7) corresponds to octet 11000111.
The octets are subdivided into control octets and data octets. The control octets more particularly indicate the headings and ends of pages, the headings and ends of rows as well as the visual display attributes. The data octets, inserted between the control octets, correspond to the characters contained in the rows. All the control or data octets incorporate the oddness bit b.sub.8.
As an example, an information page available to the user is in the following form:
(1) It firstly contains a page heading formed by control codes:
(ETX) FF RS NP.sub.1 NP.sub.2 NP.sub.3 : in which ETX is a code which terminates the preceding page, FF (0, 12) is a code indicating a new page, RS (1, 14) is a page flag, NP.sub.1, NP.sub.2, NP.sub.3 are codes indicating a page number from 001 to 999.
(2) The page heading is followed by a "zero" row:
US OO C.sub.1 C.sub.2 C.sub.3 . . . RC LF, in which US (1, 15) is a row flag, OO indicates the zero row, C.sub.1, C.sub.2, C.sub.3 are complementary codes and RC (0, 13) LF (0, 10) terminate the row.
The zero row can also contain a department name, a date, a time and optionally tax details. This zero row does not carry information constituting the service. It forms part of the procedure.
(3) The zero row is followed by information rows firstly incorporating a row heading formed by the codes US, NR.sub.1, NR.sub.2 in which US is the row flag, NR.sub.1, NR.sub.2 constitute the row number from 01 to 24, followed by the data octets d.sub.1, d.sub.2 . . . d.sub.n, which are inserted between the row heading and namely either an end of row RC, LF or an end of page ETX. These octets d.sub.1, d.sub.2 . . . d.sub.n represent the information constituting the transmitted service.
With regard to changes of the visual display attributes, they are transmitted by sequences preceded by the escape code ESC (1, 11). These attributes are the colour, the background colour, the size (double height, double width), blinking, the form of the symbols (choice of alphanumeric or semigraphical symbols), the incrustation and the masking.